The invention relates to the non-destructive checking of the thickness of the interior or exterior plating on metal tubes and more precisely to a method of and an apparatus for using ultrasonics to measure this thickness, and also to the application of this method to Zr plated alloy tubes.
The publication "Liner thickness measurement for zirconium lined Zircaloy cladding tube using dial frequency Eddy current method" by M. IWASAKI, N. SUZUKI, Y. NISHIMOTO, M. KOTANI and N. FUJII--Nuclear Engineering and Design 94 (1986)--pp. 447-452--describes a method of using Foucault currents for measuring the thickness of the internal non-alloyed Zr lining in Zircaloy tubes, this thickness being comprised between 40 um and 130 um approx. This method employs a probe which is displaced inside the tube while the tube rotates on itself. Other non-destructive measuring methods had been tried, particularly ultrasonic measuring which is "unusable because of the slight difference in acoustic impedance of Zircaloy and Zr".
Furthermore, the DERWENT abstract from the publication KOKAI JP-A-58 199 139 describes tubes comprising an external cladding of Zr alloy and an inner cladding of non-alloyed Zr, these two coatings being separated by an intermediate layer of graphite and methyl cellulose which makes it possible to use ultrasound to measure the thickness of the interior Zr cladding.
Wishing in particular to measure the thickness of the plating on Zr alloy tubes which are plated internally with non-alloyed Zr by metallurgical methods, these tubes typically having an outside diameter between 28 and 110 mm and an internal Zr plating thickness between 0.5 and 2 mm and constituting blanks for conversion to sheathing tubes, we noticed that since the use of Foucault currents for testing the interior of tubes required operations to achieve introduction into and then extraction from the interior of the tube to be checked, series inspection would be complicated and would not be very sensitive or would be difficult where the plating thicknesses exceeded 1 mm.
Notwithstanding the unfavourable indications in the first document and the supporting information provided by the second document, the we sought to perfect a non-destructive method of measuring the thickness of the interior lining from outside the tubes in question and wondered whether ultrasonic inspection would not nevertheless be possible.